arXiv subjects

Niclas Tilgner

Publications and source records attributed to Niclas Tilgner.

8 recordsLinked to original sources

arXiv2026

Spin- and Angle-resolved Photoelectron Spectroscopy Study of the Quantum Spin Hall Insulator Bismuthene and its Precursor Phase

Recent studies have revealed that a confined bismuth layer at the graphene/SiC interface can be reversibly switched between a topologically trivial precursor phase and the quantum spin Hall insulator bismuthene. Here, we present a detailed spin- and angle-resolved photoelectron spectroscopy study of both structures, resolving the spin texture of their low-energy electronic states. Owing to the strong intrinsic spin-orbit coupling of bismuth and the asymmetric confinement potential at the interface, the valence bands of both structures are Rashba-split. We demonstrate the expected spin-momentum locking for both phases and Kramers' doublets through the investigation of the valence bands' spin polarization at multiple positions in reciprocal space.

cond-mat.mtrl-sci↗

arXiv2026

Ultrafast signatures of Dirac / flat-band hybrid states from time-resolved ARPES

Hybridization of highly itinerant Dirac electrons with localized flat-band states is predicted to yield emergent phenomena such as exotic heavy-fermion behaviour. Epitaxial graphene on two-dimensional adsorbate structures on SiC(0001), which host flat bands, offers a promising platform to explore these effects. However, direct experimental evidence of interlayer hybridization in such systems has so far been lacking. Here, we address this gap using time- and angle-resolved photoemission spectroscopy (trARPES) where interlayer hybridization manifests in three key observations: (1) accelerated Dirac-carrier relaxation arising from additional electronic and phononic decay channels provided by the flat-band subsystem, (2) transient charging of the Dirac cone enabled by direct optical excitation from the flat bands, and (3) ultrafast back-transfer of charge into the flat bands on timescales governed by the interlayer coupling strength. We further demonstrate that the degree of hybridization can be tuned via the atomic number of the atoms intercalated at the graphene-SiC interface, establishing a controllable platform for investigating exotic correlated ground states.

cond-mat.mtrl-sci↗

arXiv2026

Graphene-capped bismuthene on SiC as a platform for correlated quantum spin Hall edge states

Epitaxial bismuthene on SiC(0001) hosts symmetry-protected metallic edge states within a large bulk band gap, establishing it as a promising two-dimensional topological insulator for hightemperature quantum spin Hall (QSH) transport. Here we realize bismuthene islands by intercalating Bi beneath zero-layer graphene on SiC(0001) followed by hydrogen treatment, yielding well-defined edges with controlled terminations. Spectroscopic measurements demonstrate that the edge states reside inside the bulk band gap and remain charge neutral. The graphene overlayer interacts only weakly with the bismuthene, preserving its topological character while providing environmental protection. Notably, the one-dimensional edge channels exhibit signatures of enhanced electronic correlations relative to freestanding bismuthene, suggesting proximity-induced modification of the QSH edge physics. These results establish graphene-capped bismuthene as a robust and tunable platform for correlated quantum spin Hall states.

cond-mat.mes-hall↗

arXiv2026

Confinement Epitaxy of Large-Area Two-Dimensional Sn at the Graphene/SiC Interface

Confinement epitaxy beneath graphene stabilizes exotic material phases by restricting vertical growth and altering lateral diffusion, conditions unattainable on bare substrates. However, achieving long-range interfacial order while maintaining high-quality graphene remains a significant challenge. Here, we demonstrate the synthesis of large-area quasi-free-standing monolayer graphene (QFMLG) via the intercalation of a two-dimensional (2D) Sn. While the triangular Sn(1x1) interface exhibits a robust metallic band structure, the decoupled QFMLG maintains charge neutrality, confirmed by photoemission spectroscopy. Using high-resolution Raman spectroscopy and microscopy, we distinguish between direct intercalation and diffusion-driven expansion, identifying the latter as the critical pathway to superior QFMLG crystalline quality. Temperature-dependent analysis reveals dynamical structural coupling between the decoupled QFMLG and the Sn interface, providing a novel degree of freedom for strain engineering. Beyond uncovering the diffusion-driven mechanism, this work establishes metal intercalation as an effective strategy for tailoring durable graphene-metal heterostructures with tunable properties for next-generation quantum materials platforms.

cond-mat.mes-hall↗

arXiv2025

Si Intercalation Beneath Epitaxial Graphene: Modulating Mott States at the SiC(0001) Interface

Intercalation has proven to be a powerful tool for tailoring the electronic properties of freestanding graphene layers as well as for stabilizing the intercalated material in a two-dimensional configuration. This work examines Si intercalation of epitaxial graphene on SiC(0001) using three preparation methods. Dangling bond states at the interface were found to undergo a Mott-Hubbard metal-insulator transition as a result of a significant on-site repulsion. Comparing this heterostructure consisting of graphene and a Mott insulator with a similar system without graphene, reveals the screening ability of graphene's conduction electrons on the on-site repulsion. The system presented here can serve as a template for further research on Mott insulators with variable band gap.

cond-mat.mtrl-sci↗

arXiv2025

Solving the Phase Problem of Diffraction: X-ray Standing Waves Imaging on Bismuthene/SiC(0001)

The phase retrieval problem is a fundamental shortcoming of all diffraction-based methods, arising from the inability to measure the phase of scattered waves. The (normal incidence) X-ray standing wave (NIXSW) technique circumvents this issue by introducing a (Bragg-generated) X-ray standing wave field throughout the sample, relative to which any atomic species can be localized by probing its fluorescence or photoelectron yield. In essence, in a single measurement the complex scattering factor (i.e., its amplitude \textit{and} phase) corresponding to the used Bragg reflection is determined. Performing this for multiple Bragg reflections enables one to reconstruct the scattering density of the sample in three dimensions, straightforwardly as the Fourier sum of all measured (complex) scattering factors. Here, we utilize this technique to reveal the structural key features involved in the formation of the quantum spin Hall insulator bismuthene on silicon carbide. In this prominent example, the two-dimensional Bi layer is confined between a 4H-SiC substrate crystal and an epitaxial graphene layer. The key finding is a change in the adsorption site of the Bi atoms underneath the graphene upon hydrogenation, caused by the H-saturation of one (out of three) Si dangling bonds per unit cell. This structural change, clearly revealed by our NIXSW imaging experiment, is the key feature leading to the formation of the characteristic band structure of the 2D bismuthene honeycomb.

cond-mat.mes-hall↗

arXiv2025

Growth dynamics of graphene buffer layer formation on ultra-smooth SiC(0001) surfaces

In this study the growth process of epitaxial graphene on SiC was investigated systematically. The transition from the initial buffer layer growth to the formation of the first monolayer graphene domains was investigated by various techniques: atomic force microscopy, low energy electron diffraction, low energy electron microscopy, Raman spectroscopy, scanning tunneling spectroscopy and scanning electron microscopy. The data show that the buffer layer formation goes along with a simultaneous SiC decomposition which takes place as a rapid step retraction of one specific type of SiC bilayer in good agreement with the step retraction model. Once the buffer layer coverage is completed, the resulting characteristic regular repeating terrace and step height pattern of one and two SiC bilayers turned out to be very stable against further SiC decomposition. The following initial growth of monolayer graphene domains occurs, interestingly, only along the two bilayer high terrace edges. This behavior is explained by a preferential SiC decomposition at the higher step edges and it has some potential for spatial graphene growth control. The corresponding earlier graphene growth on one terrace type can explain the different scanning tunneling spectroscopy nanoscale resistivities on these terraces.

cond-mat.mtrl-sci↗

arXiv2025

Reversible Switching of the Environment-Protected Quantum Spin Hall Insulator Bismuthene at the Graphene/SiC Interface

Quantum Spin Hall Insulators (QSHI) have been extensively studied both theoretically and experimentally because they exhibit robust helical edge states driven by spin-orbit coupling and offer the potential for applications in spintronics through dissipationless spin transport. However, to realize devices, it is indispensable to gain control over the interaction of the active layer with the substrate, and to protect it from environmental influences. Here we show that a single layer of elemental Bi, formed by intercalation of an epitaxial graphene buffer layer on SiC(0001), is a promising candidate for a QSHI. This layer can be reversibly switched between an electronically inactive precursor state and a ``bismuthene state'', the latter exhibiting the predicted band structure of a true two-dimensional bismuthene layer. Switching is accomplished by hydrogenation (dehydrogenation) of the sample, i.e., a partial passivation (activation) of dangling bonds of the SiC substrate, causing a lateral shift of Bi atoms involving a change of the adsorption site. In the bismuthene state, the Bi honeycomb layer is a prospective QSHI, inherently protected by the graphene sheet above and the H-passivated substrate below. Thus, our results represent an important step towards protected QSHI systems beyond graphene.

cond-mat.mes-hall↗